The present invention relates to a process for the electrochemical roughening of aluminum for use in printing plate supports, the process being performed by means of an altrnating current, preferably in an electrolyte containing sulfuric acid, chloride ions and aluminum ions.
Printing plates (this term referring to offset-printing plates, within the scope of the present invention) usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction layer arranged thereon, the layer being applied to the support either by the user (in the case of plates which are not pre-coated) or by the industrial manufacturer (in the case of precoated plates).
As a layer support material, aluminum or alloys thereof have gained general acceptance in the field of printing plates. In principle, it is possible to use these supports without modifying pretreatment, but they are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process (sometimes also called graining or etching in the literature), a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In modern continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or pre-coated printing plates, a combination of the aforementioned modifying methods is frequently used, particularly a combination of electrochemical roughening and anodic oxidation, optionally followed by a hydrophilizing step.
Roughening is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3 or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, using an alternating current. The peak-to-valley heights (specified, for example, as mean peak-to-valley heights R.sub.z) of the roughened surface, which can thus be obtained, are in the range from about 1 to 15 .mu.m, particularly in the range from about 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768 (in the October 1970 version). The peak-to-valley height R.sub.z is then the arithmetic mean calculated from the individual peak-to-valley height values of five mutually adjacent individual measurement lengths.
Roughening is, inter alia, carried out in order to improve the adhesion of the reproduction layer to the support and to improve the water/ink balance of the printing form which results from the printing plate upon irradiating (exposure) and developing. By irradiating and developing (or decoating, in the case of electrophotographically-working reproduction layers), the ink-receptive image areas and the water-retaining non-image areas (generally the bared support surface) in the subsequent printing operation, are produced on the printing plate, and thus the actual printing form is obtained. The final topography of the aluminum surface to be roughened is influenced by various parameters.
The paper "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the roughening of aluminum in aqueous solutions of hydrochloric acid, based on variations of the following process parameters and an investigation of the corresponding effects. The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable by means of the pH) and in view of the Al.sup.3+ ion concentration, with influences on the surface topography being observed. Temperature variations between 16.degree. C. and 90.degree. C. do not show an influence causing changes until temperatures are about 50.degree. C. or higher, the influence becoming apparent, for example, as a significant decrease in layer formation on the surface. Variations in roughening time between 2 and 25 minutes lead to an increasing metal dissolution with increasing duration of action. Variations in current density between 2 and 8 A/dm.sup.2 result in higher roughness values with rising current density. If the acid concentration is in a range from 0.17% to 3.3% of HCl, only negligible changes in pit structure occur between 0.5% and 2% of HCl, whereas below 0.5% of HCl, the surface is only locally attacked, and at high values, an irregular dissolution of aluminum takes place. If a direct current is used instead of an alternating current it appears that, obviously, both types of half-waves are necessary to achieve uniform roughening. It is already pointed out in the above-mentioned paper that the addition of sulfate ions increasingly produces undesired, coarse, non-homogenous roughening structures which are unsuitable for lithographic purposes.
The use of hydrochloric acid in the roughening of aluminum substrates is thus to be considered as being basically known in the art. A uniform graining can be obtained, which is appropriate for lithographic plates and is within a useful roughness range. In pure hydrochloric acid electrolytes, adjustment of an even and uniform surface topography is difficult and it is necessary to keep the operating conditions within very close limits.
The influence of the electrolyte composition on the quality of roughening is, for example, also described in the following publications:
German Offenlegungsschrift No. 22 50 275 (=British Patent Specification No. 1,400,918) specifies aqueous solutions containing from 1.0% to 1.5% by weight of HNO.sub.3 or from 0.4% to 0.6% by weight of HCl and optionally from 0.4% to 0.6% by weight of H.sub.3 PO.sub.4, for use as electrolytes in the roughening of aluminum for printing plate supports, by means of an alternating current, and
German Offenlegungsschrift No. 28 10 308 (=U.S. Pat. No. 4,072,589) mentions aqueous solutions containing from 0.2% to 1.0% by weight of HCl and from 0.8% to 6.0% by weight of HNO.sub.3 as electrolytes in the roughening of aluminum with an alternating current.
Additives used in the HCl electrolyte serve the purpose of preventing an adverse local attack in the form of deep pits. The following additives to hydrochloric acid electrolytes are, for example, described:
in German Offenlegungsschrift No. 28 16 307 (=U.S. Pat. No. 4,172,772): monocarboxylic acids, such as acetic acid, PA1 in U.S. Pat. No. 3,963,594: gluconic acid, PA1 in European Patent Application No. 0 036 672: citric acid and/or malonic acid, and PA1 in U.S. Pat. No. 4,052,275: tartaric acid.
All these organic electrolyte components have the disadvantage of being electrochemically unstable and of decomposing in the case of a high current load (voltage).
Inhibiting additives, for example, phosphoric acid and chromic acid as described in U.S. Pat. No. 3,887,447 or boric acid as described in German Offenlegungsschrift No. 25 35 142 (=U.S. Pat. No. 3,980,539) have the disadvantage that there is often a local breakdown of the protective effect and individual, particularly pronounced pits can form in these places.
Japanese Patent Application Disclosure No. 17580/80 describes roughening by means of an alternating current in a composition comprising hydrochloric acid and an alkali-metal halide to produce a lithographic support material.
German Offenlegungsschrift No. 16 21 115 (=U.S. Pat. Nos. 3,632,486 and 3,766,043) describes roughening by means of a direct current, for example, for decorative panellings, using dilute hydrofluoric acid, the aluminum being switched such that it forms the cathode.
German Patent No. 120 061 describes a treatment for generating a hydrophilic layer by the application of electric current, which treatment can also be performed in hydrofluoric acid.
German Offenlegungsschrift No. 29 34 597 (=U.S. Pat. Nos. 4,201,836, 4,242,417 and 4,324,841) describes an optionally electrochemical roughening of aluminum, using a saturated aluminum salt solution which may additionally be admixed with up to 10% of a mineral acid. The examples given are based on aluminum chloride as the salt and hydrochloric is optionally added.
A saturated aluminum chloride solution of this kind (&gt;500 g/l of AlCl.sub.3 .times.6H.sub.2 O), in particular in the acidic region, represents an acute corrosion hazard to the materials used. Specifically, the surface quality obtainable with sulfuric acid as the mineral acid added, which is, however, not described in the examples, would be very pitted and thus unsuitable for lithographic applications, as shown by Comparative Examples C24 to C33.
Japanese Patent Publication No. 006571/76 describes roughening of an aluminum sheet for lithographic printing plates, using an alternating current in electrolytes containing from 1% to 4% of HCl and from 0.1% to 1% of H.sub.2 SO.sub.4. As shown by Comparative Examples C34 to C53, the surface profiles obtainable in this range of concentration of the electrolyte show an irregular roughening and are not in accordance with the state of the art.
In British Patent No. 1,392,191, the influence of sulfate ions present in concentrations of more than 10 to 15 ppm in hydrochloric acid electrolytes used in the preparation of a lithographic support material, is described as being detrimental and, to overcome this difficulty, an addition of phosphoric acid is employed.
According to European Patent Application No. 0 132 787 aluminum for use as a support material for printing plates is roughened in 1,000 to 40,000 ppm of nitric acid containing from 50 to 4,000 ppm (up to 0.4%) of sulfate ions; also in this case, the detrimental influence of higher concentrations is mentioned. It is stated that over 5,000 ppm roughening is even prevented.
In U.S. Pat. No. 1,376,366, an electrochemical treatment of metals, in particular steel, is described, in which direct current is used in a solution comprising ammonium chloride, sulfuric acid and nitric acid. In this process, a shaping treatment of a workpiece is attempted. A roughening treatment for lithographic surfaces, on the other hand, is intended to produce a very fine (1 to 10 .mu.m), coat-free structuring of the surface, by which good anchoring of the copying layer and retaining of the dampening solution during the printing process is to be ensured. Formation of a coat during roughening can be suppressed by the application of an alternating current.
U.S. Pat. No. 3,284,326 describes roughening of an aluminum foil for use in the manufacture of capacitors. In the process direct current is employed to achieve a high capacitance. The electrolyte used comprises a solution of chloride and phosphate, the type of the cation--with the exception of the disadvantageous aluminum--being insignificant in view of the roughening of the capacitor foil. Up to 10 mol-% of the cation can also be replaced by H.sup.+ ; it is, however, pointed out in the specification that it is not good to start the process with an acid-containing electrolyte.
According to the following publications, roughening of aluminum for use as a capacitor foil is carried out in systems containing aluminum chloride and sulfate: U.S. Pat. No. 4,427,506, U.S. Pat. No. 4,395,305, Japanese Patent Application Disclosure No. 76100/80, Japanese Patent Publication No. 39169/78, Japanese Patent Application Disclosure No. 141444/77 and Japanese Patent Publication No. 25142/74.
In contrast to the sole object of producing a marked surface enlargement in foils for use in capacitors, the basically different roughening employed for printing plate supports serves to improve the anchoring of the copying layer and the water/ink balance and must therefore be very homogeneous and pit-free within a narrow range of peak-to-valley heights.
In U.S. Pat. No. 4,427,506 it is pointed out that in connection with the manufacture of capacitor foils a content of sulfate ions &gt;500 ppm is detrimental.
Another known possibility of improving the uniformity of electrochemical roughening comprises a modification of the type of electric current employed, including, for example,
using an alternating current in which the anodic voltage and the anodic coulombic input are higher than the cathodic voltage and the cathodic coulombic input, according to German Offenlegungsschrift No. 26 50 762 (=U.S. Pat. No. 4,087,341), the anodic half-cycle period of the alternating current being generally adjusted to be less than the cathodic half-cycle period; this method is, for example, also referred to in German Offenlegungsschrift No. 29 12 060 (=U.S. Pat. No. 4,301,229), German Offenlegungsschrift No. 30 12 135 (=published UK Patent Application No. 2,047,274) or German Offenlegungsschrift No.30 30 815 (=U.S. Pat. No. 4,272,342), PA0 using an alternating current in which the anodic voltage is markedly increased compared with the cathodic voltage, according to German Offenlegungsschrift No. 14 46 026 (=U.S. Pat. No. 3,193,485), PA0 interrupting the current flow for 10 to 120 seconds and re-applying current for 30 to 300 seconds, using an alternating current and, as the electrolyte, an aqueous solution of 0.75 to 2.0 N HCl, with the addition of NaCl or MgCl.sub.2, according to British Patent No. 879,768. A similar process comprising an interruption of current flow in the anodic or cathodic phase is also disclosed in German Offenlegungsschrift No. 30 20 420 (=U.S. Pat. No. 4,294,672). PA0 "Pure aluminum" (DIN Material No. 3.0255), i.e., composed of more than 99.5% Al, and the following permissible admixtures (maximum total 0.5%) of 0.3% Si, 0.4% Fe, 0.03% Ti, 0.02% Cu, 0.07% Zn and 0.03% of other substances, or PA0 "Al-alloy 3003" (comparable to DIN Material No. 3.0515), i.e., composed of more than 98.5% Al, 0 to 0.3% Mg and 0.8% to 1.5% Mn, as alloying constituents, and 0.5% Si, 0.5% Fe, 0.2% Ti, 0.2% Zn, 0.1% Cu and 0.15% of other substances, as permissible admixtures. PA0 The direct current sulfuric acid process, in which anodic oxidation is carried out in an aqueous electrolyte which conventionally contains approximately 230 g of H.sub.2 SO.sub.4 per 1 liter of solution, for 10 to 60 minutes at 10.degree. C. to 22.degree. C., and at a current density of 0.5 to 2.5 A/dm.sup.2. In this process, the sulfuric acid concentration in the aqueous electrolyte solution can also be reduced to 8% to 10% by weight of H.sub.2 SO.sub.4 (about 100 g of H.sub.2 SO.sub.4 per liter), or it can also be increased to 30% by weight (365 g of H.sub.2 SO.sub.4 per liter), or more. PA0 The "hard-anodizing process" is carried out using an aqueous electrolyte, containing H.sub.2 SO.sub.4 in a concentration of 166 g of H.sub.2 SO.sub.4 per liter (or about 230 g of H.sub.2 SO.sub.4 per liter), at an operating temperature of 0.degree. to 5.degree. C., and at a current density of 2 to 3 A/dm.sup.2, for 30 to 200 minutes, at a voltage which rises from approximately 25 to 30 V at the beginning of the treatment, to approximately 40 to 100 V toward the end of the treatment. PA0 positive-working o-quinone diazide compounds, preferably o-naphthoquinone diazide compounds, which are described, for example, in German Pat. Nos. 854 890, 865 109, 879 203, 894 959, 938 233, 11 09 521, 11 44 705, 11 18 606, 11 20 273 and 11 24 817; PA0 negative-working condensation products from aromatic diazonium salts and compounds with active carbonyl groups, preferably condensation products formed from diphenylamine-diazonium salts and formaldehyde, which are described, for example, in German Pat. Nos. 596,731, 11 38 399, 11 38 400, 11 38 401, 11 42 871, and 11 54 123, U.S. Pat. Nos. 2,679,498 and 3,050,502 and British Patent No. 712,606; PA0 negative-working co-condensation products of aromatic diazonium compounds, for example, according to German Offenlegungsschrift No. 20 24 244, which possess, in each case, at least one unit of the general types A(-D).sub.n and B, connected by a divalent linking member derived from a carbonyl compound which is capable of participating in a condensation reaction. In this context, these symbols are defined as follows: A is the radical of a compound which contains at least two aromatic carbocyclic and/or heterocyclic nuclei, and which is capable, in an acid medium, of participating in a condensation reaction with an active carbonyl compound, at one or more positions. D is a diazonium salt group which is bonded to an aromatic carbon atom of A; n is an integer from 1 to 10, and B is the radical of a compound which contains no diazonium groups and which is capable, in an acid medium, of participating in a condensation reaction with an active carbonyl compound, at one or more positions on the molecule; PA0 positive-working layers according to German Offenlegungsschrift No. 26 10 842 containing a compound which, on being irradiated, splits off an acid, a compound which possesses at least one C-O-C group, which can be split off by acid (e.g., an orthocarboxylic acid ester group, or a carboxamide-acetal group), and, if appropriate, a binder; PA0 negative-working layers, composed of photopolymerizable monomers, photo-initiators, binders and, if appropriate, further additives. In these layers, for example, acrylic and methacrylic acid esters, or reaction products of diisocyanates with partial esters of polyhydric alcohols are employed as monomers, as described, for example, in U.S. Pat. Nos. 2,760,863 and 3,060,023, and in German Offenlegungsschriften Nos. 20 64 079 and 23 61 041. Suitable photo-initiators are, inter alia, benzoin, benzoin ethers, polynuclear quinones, acridine derivatives, phenazine derivatives, quinoxaline derivatives, quinazoline derivatives, or synergistic mixtures. A large number of soluble organic polymers can be employed as binders, for example, polyamides, polyesters, alkyd resins, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, gelatin or cellulose ethers; PA0 negative-working layers according to German Offenlegungsschrift No. 30 36 077, which contain, as the photosensitive compound, a diazonium salt polycondensation product, or an organic azido compound, and which contain, as the binder, a high-molecular weight polymer with alkenylsulfonylurethane or cycloalkenylsulfonylurethane side groups.
The aforementioned methods may lead to relatively uniformly roughened aluminum surfaces, but they sometimes require a comparatively great equipment expenditure and, in addition, are applicable only within closely limited parameters.